Performance

[KyleCotton]

Check the performance of the library.
Ensure the structure that the stop-words crate is providing has constant time lookup.
Consider using lazy_static for this.

[KyleCotton]

https://doc.rust-lang.org/std/collections/index.html

[KyleCotton]

It is essential for benchmarks and production to run in release mode.

cargo run --release

[KyleCotton]

Update the preprocessor, using lazy_static! or similar, so that the runtime cost is as small as possible.

[KyleCotton]

Consider not .collect::<T>() into a Vec at each stage.

[KyleCotton]

Mutate the Vec, consider using .drain.